1. Field of the Invention
The invention is directed to a method of recovering valuable phosphoric acid from residues generated during the purification of wet process phosphoric acid. The invention is specifically directed to the recovery of phosphoric acid from a magnesium acid pyrophosphate filter cake.
2. Related Art
U.S. Pat. No. 4,443,421 to Hollifield et al. discloses a process for removing particulate impurities from aqueous phosphoric acid. In the process disclosed by Hollifield et al., after filtering the phosphoric acid as described in the reference, the filter cake can be contacted with a wash fluid, as for instance water, to remove entrained phosphoric acid from the filter cake.
U.S. Pat. No. 4,409,194 to Petersen discloses a process for separating magnesium from wet process superphosphoric acid by filtration. This patent is incorporated herein by reference.
Wet process phosphoric acid is conventionally prepared by reacting sulfuric acid and phosphate rock, followed by filtration to remove insoluble gypsum and other insoluble compounds. The resulting dilute, weak phosphoric acid containing about 26-30% P.sub.2 O.sub.5 by weight, is commonly known as "filter acid" and is a highly impure material containing the dissolved sulfates, fluorosilicates, and salts of iron, aluminum, magnesium, sodium and other metals. These impurities may precipitate and settle out in varying rates and amounts during storage or further processing of the dilute wet phosphoric acid.
Concentrations of weak, wet-process phosphoric acid up to the superphosphate range (containing 64-72% P.sub.2 O.sub.5) is done in two steps. Preferably, this two-step concentration is done in separate equipment because of variations in temperature, corrosion and viscosity that occur over the total range. As a first step, it is common to evaporate a dilute or weak acid and to partially purify the acid by removal of precipitated impurities consisting of CaSO.sub.4, Na.sub.2 SiF.sub.6, (Fe,Al).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O, and other salts to a concentration of about 38 to about 56 weight percent P.sub.2 O.sub.5. This acid is known as "evaporator" acid, with about 54% P.sub.2 O.sub.5 being a most common strength.
As a second step, the partially purified evaporated acid (38-56 weight percent P.sub.2 O.sub.5) is further evaporated to superphosphoric acid containing about 64-72 weight percent P.sub.2 O.sub.5. Impurities that precipitate in the production of the superphosphoric acid consist of MgH.sub.2 P.sub.2 O.sub.7, FeH.sub.2 P.sub.3 O.sub.10, AlH.sub.2 P.sub.3 O.sub.10 and other salts.
Liquid ammonium phosphate fertilizer solutions are derived from purified wet process superphosphoric acid. Said solutions, commonly 10-34-0 grade (10 weight percent N, 34 weight percent P.sub.2 O.sub.5 and 0 weight percent K.sub.2 O), and other variations, are prepared either (1) by reacting superphosphoric acid containing 64%-72% P.sub.2 O.sub.5 with liquid and/or gaseous ammonia or (2) by reacting acid containing 54 to 60 weight percent P.sub.2 O.sub.5 with gaseous ammonia.
U.S. Pat. No. 4,409,194 discloses a process for removing magnesium impurities from the wet process acid. The process disclosed is to culture and precipitate magnesium from wet process superphosphoric acid in the form of singular, well-defined crystals, as well as some agglomerates. That process comprised aging wet process superphosphoric acid containing 62%-72% by weight of P.sub.2 O.sub.5 with about 10%-45% of the P.sub.2 O.sub.5 in the polyphosphate form, from about 4 to 80 hours with intermittent or no agitation. Thereafter, the aged acid from which magnesium has crystallized in the form of MgH.sub.2 P.sub.2 O.sub.7 is filtered to remove the acid therefrom. In this filtration process, as well as in the filtration process associated with removing magnesium impurities from wet process acid, a significant quantity of free phosphoric acid is retained in the filter cake.
It is an object of the instant invention to provide a means for recovering the free phosphoric acid. It is also an object of the invention to provide a magnesium polyphosphate product suitable for use as a fertilizer or for other purposes. In addition, it is a further object of the invention to recover the free acid in a form suitable for use as a fertilizer or industrial chemical feed stock.